It sounds to me like you’re on the right track with what you’re aiming for.
A little known fact is that the carbon they use to make black ink is not perfectly neutral. At least, when they ground it as fine as they need to for inkjet inks, it starts to go brown. So RIPs would expect to add CMY into a black ramp at a certain point. Now, they may have solved this problem with more modern ink sets - but I wouldn’t be surprised if ONYX still starts a little bit of CMY early on in its algorithms as you see.
This is probably one of those things that should not be, but are. Right up there with why a strong red build includes a small amount of cyan ink. And how come glue doesn’t stick to the inside of the bottle? And who created the word “Lisp” to have an “s” in it? I dunno. Some things in life are just wrong. 